This invention relates to electrophotography and more particularly, to an improved method of fabricating an overcoated electrophotographic imaging member.
Generally, electrophotographic imaging processes involve the formation and development of electrostatic latent images on the imaging surface of a photoconductive member. The photoconductive member is usually imaged by uniformly electrostatically charging the imaging surface in the dark and exposing the member to a pattern of activating electromagnetic radiation such as light, to selectively dissipate the charge in the illuminated areas of the member to form an electrostatic latent image on the imaging surface. The electrostatic latent image is then developed with a developer composition containing toner particles which are attracted to the photoconductive member in image configuration. The resulting toner image is often transferred to a suitable receiving member such as paper.
The photoconductive members include single or multiple layered devices comprising homogeneous or heterogeneous inorganic or organic compositions and the like. One example of a photoconductive member containing a heterogeneous composition is described in U.S. Pat. No. 3,121,006 wherein finely divided particles of a photoconductive inorganic compound is dispersed in an electrically insulating organic resin binder. The commercial embodiment usually comprises a paper backing containing a coating thereon of a binder layer comprising particles of zinc oxide uniformly dispersed therein. Useful binder materials disclosed therein include those which are incapable of transporting for any significant distance injected charge carriers generated by the photoconductive particles. Thus, the photoconductive particles must be in substantially contiguous particle to particle contact throughout the layer for the purpose of permitting charge dissipation required for cyclic operation. Thus, about 50 percent by volume of photoconductive particles is usually necessary in order to obtain sufficient photoconductive particle to particle contact for rapid discharge. These relatively high photoconductive concentrations can adversely affect the physical continuity of the resin binder and can significantly reduce the mechanical strength of the binder layer.
Other known photoconductive compositions include amorphous selenium, halogen doped amorphous selenium, amorphous selenium alloys including selenium arsenic, selenium tellurium, selenium arsenic antimony, halogen doped selenium alloys, cadmium sulfide and the like. Generally, these inorganic photoconductive materials are deposited as a relatively homogeneous layer on suitable conductive substrates. Some of these inorganic layers tend to crystallize when exposed to certain vapors that may occasionally be found in the ambient atmosphere. Moreover, the surfaces of selenium type photoreceptors are highly susceptible to scratches which print out in final copies.
Still other electrophotographic imaging members known in the art comprise a conductive substrate having deposited thereon an organic photoconductor such as a polyvinylcarbazole-2,4,7-trinitrofluorenone combination, phthalocyanines, quinacridones, pyrazolones and the like. Some of these photoreceptors, such as those containing 2,4,7-trinitrofluorenone, present health or safety issues.
Recently, there has been disclosed layered photoresponsive devices comprising photogenerating layers and transport layers deposited on conductive substrates as described, for example, in U.S. Pat. No. 4,265,990 and overcoated photoresponsive materials containing a hole injecting layer, a hole transport layer, a photogenerating layer and a top coating of an insulating organic resin, as described, for example, in U.S. Pat. No. 4,251,612. Examples of photogenerating layers disclosed in these patents include trigonal selenium and various phthalocyanines and hole transport layers containing certain diamines dispersed in inactive polycarbonate resin materials. The disclosures of each of these patents, namely, U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,251,612 are incorporated herein by reference in their entirety. Other representative patents containing layered photoresponsive devices include U.S. Pat. No. 3,041,116; U.S. Pat. No. 4,115,116; U.S. Pat. No. 4,047,949 and U.S. Pat. No. 4,081,274. These patents relate to systems that require negative charging for hole transporting layers when the photogenerating layer is beneath the transport layer. Photogenerating layers overlying hole transport layers require positive charging but must be equal to or less than about 1 to 2 micrometers for adequate sensitivity and therefore wear away quite rapidly.
While the above described electrophotographic imaging members may be suitable for their intended purposes, there continues to be a need for improved devices. For example, the imaging surface of many photoconductive members is sensitive to wear, ambient fumes, scratches and deposits which adversely affect the electrophotographic properties of the imaging member.
Also, in multilayered photoreceptors comprising a charge generating layer and a charge transport layer, wear of the transport layer during image cycling limits the life of small diameter organic photoreceptor drums employed in copiers, duplicators, printers, facsimile machines and the like. With the advent of Bias Charging Rolls (BCR),and Bias Transfer Rolls (BTR) the drum wear is catastrophic. Even with the gentlest of the Bias Charging Rolls, the wear is as much as 8 to 10 micrometers in 100 kilocycles of revolutions. With the small diameter drum and duty cycle considerations 100 kilocycles of revolution translates to as little as 10,000 to 20,000 prints. The machines employing these small diameter drums do not employ exposure control. Wear results in considerable reduction of sensitivity of the device. A drum life of 50,000 or more prints (one or million drum revolution cycles) is sorely needed.
Overcoating layers have been proposed to overcome the undesirable characteristics of uncoated photoreceptors. However, many of the overcoating layers adversely affect electrophotographic performance of an electrophotographic imaging member. Moreover, the application of an overcoat requires an additional coating and drying step which increases the number of processing steps and increases fabrication costs. One way of reducing cost (of plant as well as manufacturing process), would be to skip the transport layer drying step. In this scheme, after the transport layer is coated (by dip or other processes), the overcoat is coated and then both transport layer and overcoat are dried in one step to increase throughput. However, in the one step drying process, the overcoat can harden before the transport layer solvent is adequately removed and high residual solvent content in the generator and transport layers severely affects the shape of the photoinduced discharge curve (PIDC) during imaging. Moreover, application of an overcoat composition that transports holes (without trapping), is insensitive to moisture, has a low wear rate and can be applied without redissolving the transport layer is not a simple task. While some of the above-described imaging members exhibit certain desirable properties such as protecting the surface of an underlying photoconductive layer, there continues to be a need for improved overcoating layers for protecting electrophotographic imaging members.